J . Phys. Chem. 1989, 93, 5958-5960
5958
Copper( I I ) Phthalocyanine: Electronic and Vibrational Tunneling Spectra K. W. Hipps Department of Chemistry. Washington State University, Pullman, Washington 99164-4630 (Receioed: May I I, 1989)
Inelastic electron tunneling spectra (IETS) obtained from AI-AI0,-CuPc-M junctions (M = Pb or TI) are presented and compared with previous reports. Improved experimental methods allow us to report the entire spectrum in the region below 16000 cm-I in both bias directions. In contrast to previous studies, we will show that (a) tunneling spectra are very dependent upon the AIO,/CuPc and CuPc/M imbedded interfaces, (b) spectra contain both temperature-dependent and temperature-independent features, and (c) certain electronic and the vibrational features depend on junctionabias.
Introduction
The study of solid-solid interfaces imbedded within a complex structure is of great theoretical and experimental importance. Such imbedded interfaces occur in integrated circuits, in MIS and MIM sensors, and in many other solid-state devices. T o study these interfaces by spectroscopic methods, without first destroying them, is extremely difficult. There exists, however, a spectroscopic method that requires the existence of a solid-solid interfaceinelastic electron tunneling spectroscopy (IETS). Tunneling spectroscopy has been used to study electronic and vibrational states of M'-I-X-M (metal-insulator-add layer-metal) junctions.'-" It is natural, therefore, to contemplate the application of IETS to the study of imbedded interfaces of the type found in electronic devices. Metal phthalocyanine (MPc) films in contact with a metal oxide on one face and a metal at the other face occur with great frequency in the l i t e r a t ~ r e . ' ~ .These '~ devices are of interest as chemical sensors, photoconductors, and redox agents. Thus, the tunneling spectra obtained from these devices can provide valuable insights into the atomic scale processes occurring a t the MPc's imbedded interfaces. Tunneling spectra for AI-AI0,-MPc-Pb junctions have been reported several times.'-6 These are important papers because they established that electronic transitions could be observed by IETS. I n every case, however, there were troubling aspects about, and inconsistencies between, the reported MPc spectra. First and foremost, the spectra did not have the expected bias voltage dependent symmetry. As is predicted theoretically,6 and as we have demonstrated experimentally for electronic and vibrational transitions," bands seen with A1 biased negatively should also occur when AI is biased positively (somewhat reduced in intensity). MPc spectra often do not show this symmetry in the reported electronic spectra, and no bias-dependent vibrational data have been reported.'-5 I n one case,4 the authors even observed scan history dependent bands for aluminum biased positively. Despite this, all the literature reports assert that both bias data setsI4 are
( I ) Leger, A.; Klein, J . ; Belin. M.; Defourneau, D. Solid State Commun. 1972. 11. 1331. (2) de Cheveigne, S.; Klein, J.; Leger, A,; Belin, M.; Defourneau, D. Phys. Reu. B 1977, 15. 750. (3) Roll, U . ; Ewert, S.; Luth, H. Chem. Phys. Left. 1978, 58. 91 (4) Luth, H.; Roll, U.; Ewert, S . Phys. Rec-. B 1978. 18. 4241. ( 5 ) Ewert, S. Appl. Phys. A 1981, 26, 63. (6) Hansma, P. K. Tunneling Spectroscopy; Plenum Press: New York, 1982. ( 7 ) Hipps, K . W.; Mazur, U . J . Am. Chem. Soc. 1987, 109. 3861. (8) Hipps, K. W. Nature 1987, 326, 107. (9) Hipps, K. W.; Mazur, U . J . Phys. Chem. 1987, 91, 5218. ( I O ) Hipps, K . W . ; Mazur, ti. Rec. Sci. Instrum. 1988, 59, 1903. ( 1 I ) Hioos. K. W.: Mazur. U. S u r f . Sci. 1989. 207. 385.
true and correct spectra for the MPc incorporated into the M I M junction. The published intensity data are in the form of (d2V/d12)3-5 or as very low resolution (l/u)(du/dV) data.Is2 Thus, the authors found it necessary to segment their data collection. They provided limited vibrational data under one set of experimental conditions, and hard to discern electronic transitions were reported under other conditions. Large regions of the tunneling spectra were not reported. Since only rough ranges of CuPc film thickness were provided (1-3 nm1S2and 1-10 nm3-5), it is not possible to directly associate the reported vibrational and electronic state data. W e further note that only spectra from P b top metal junctions have actually been published. Thus, many questions about the MPc/M interface remained unanswered. In this initial report on CuPc-containing devices, we demonstrate that the properties of these imbedded MPc layers are much more complicated than indicated by previous authors. Our intent here is not to "solve" the CuPc tunneling spectrum, but rather to demonstrate that it has not been solved and deserves further study. Experimental Section
Metals were >99.999% purity and the CuPc was purchased from several sources and multiply sublimed. Metals were resistively deposited from W wire (AI) or from M o boats (Pb, TI). The CuPc was deposited from quartz tubes, from A120, crucibles, or from W (ME-I from R. D. Mathis Inc.). The observed spectra did not depend on the choice of source. The CuPc was deposited at a rates ranging from 0.05 to 0.2 nm/s as measured by a quartz crystal film thickness monitor. The reported CuPc thickness is accurate to about 30%. All spectra were measured in constant resolution mode.I0 That is they are normalized tunneling intensities'O (NTI). The high bias data presented here are significantly better than those given in previous reports.'-5 Each reported spectrum is the sum of from 4 to 64 repeated scans. Results and Discussion
In our early efforts to reproduce published CuPc spectra, we used standard methods that have yielded high-quality tunneling spectra for a very wide range of materials.6-'' In a single conTorr, grew an oxide tinuous process we deposited AI a t
